Use bracket syntax [qualifier].[name]

Author: alex-spies

x-pack/plugin/esql-core/src/main/java/org/elasticsearch/xpack/esql/core/expression/Attribute.java

@@ -85,7 +85,7 @@ public String qualifier() {
     }
 
     public String qualifiedName() {
-        return qualifier != null ? qualifier + " " + name() : name();
+        return qualifier != null ? "[" + qualifier + "].[" + name() + "]" : name();
     }
 
     @Override

x-pack/plugin/esql/src/main/antlr/EsqlBaseLexer.tokens

@@ -151,20 +151,20 @@ aggField
     ;
 
 qualifiedName
-    : {this.isDevVersion()}? qualifier=UNQUOTED_IDENTIFIER name=unqualifiedName
-    | name=unqualifiedName
+    : {this.isDevVersion()}? OPENING_BRACKET qualifier=UNQUOTED_IDENTIFIER CLOSING_BRACKET DOT OPENING_BRACKET name=fieldName CLOSING_BRACKET
+    | name=fieldName
     ;
 
-unqualifiedName
+fieldName
     : identifierOrParameter (DOT identifierOrParameter)*
     ;
 
 qualifiedNamePattern
-    : {this.isDevVersion()}? qualifier=ID_PATTERN name=unqualifiedNamePattern
-    | name=unqualifiedNamePattern
+    : {this.isDevVersion()}? OPENING_BRACKET qualifier=ID_PATTERN CLOSING_BRACKET DOT OPENING_BRACKET name=fieldNamePattern CLOSING_BRACKET
+    | name=fieldNamePattern
     ;
 
-unqualifiedNamePattern
+fieldNamePattern
     : (identifierPattern (DOT identifierPattern)*)
     ;